.box {
  width: 0;
  height: 0;
  /* background-color: blueviolet; */
  padding: 0;
  /* border: 20px solid red; */
  margin: 40px;
  /* box-sizing: border-box; */
  /* box-sizing: content-box; */
  border-top: 20px solid white;
  border-right: 20px solid white;
  border-bottom: 20px solid yellow;
  border-left: 20px solid yellow;
}
.box2 {
  width: 300px;
  height: 100px;
  background-color: red;
  border: 10px solid red;
  border-radius: 50px;
}